1. Field of the Invention
The present invention relates to a test apparatus, and particularly to a technique for testing a device under test which transmits/receives a differential signal.
2. Description of the Related Art
In order to transmit/receive signals among semiconductor devices, a differential signal is employed. For example, XDR-DRAM (eXtreme Data Rate Dynamic Random Access Memory) transmits a differential signal at a high speed in a bidirectional manner using a single differential transmission line. In the case of testing a device having such a bidirectional differential interface, a test is performed in which the amplitude of a differential signal output from a device under test (DUT) is measured so as to judge the quality of the DUT.
Discussion will be made regarding a test apparatus which tests a DUT having a bidirectional differential interface. The test apparatus includes a transmitter and a receiver connected to a common differential transmission line pair (which will also be referred to as simply a “differential transmission line” hereafter). The transmitter transmits a test pattern to the DUT. The receiver judges the logical value of a differential signal pair (which will also be referred to as simply a “differential signal” hereafter), or checks the amplitude of the difference in voltage between the signals in the differential signal pair.
The receiver of the test apparatus is connected to the DUT via the differential transmission line pair. With such an arrangement, the receiver is also connected to the transmitter on the test apparatus side. Accordingly, there is a need to design such a DUT test apparatus including a bidirectional differential interface so as to protect the receiver thereof from being affected by the output of the transmitter arranged adjacent to the receiver. Related techniques are described in Patent documents 1 through 3. In particular, Patent document 1 discloses a circuit (hybrid circuit) in which, in a bidirectional communication, the circuit receives only a signal from the other circuit by canceling out the transmission signal output from the former circuit itself.
[Patent Document 1]
Japanese Patent Application Laid Open No. S47-11702
[Patent Document 2]
Japanese Patent Application Laid Open No. H08-23354
[Patent Document 3]
Japanese Patent Application Laid Open No. 2006-23233
[Patent Document 4]
U.S. Pat. No. 6,573,764 B1
[Patent Document 5]
U.S. Pat. No. 7,190,194 B2